Point-by-point measurements of relative optical path length displacements can be made over a wide range of such optical displacements by measuring rates of interferometric phase variation with wavenumber. For example, a spatially coherence source beam composed of multiple wavelengths, i.e., a low temporally coherent beam, can be divided by a beamsplitter into an object beam that is reflected from the test object and a reference beam that is reflected from a reference reflector. Reflected light from both the test object and the reference reflector is recombined at a beamsplitter into a measurement beam and refocused within a detector such as a spectrometer, which records interference intensities of the different spectral components of the returning measurement beam. Based on the linear relationship between (a) the rate of change in interference phase with the change in beam frequency, referred to a modulation frequency, and (b) the optical path length difference between the object and reference beams, the relative optical displacements between different measured points can be ascertained.
Since information is collected on a point-by-point basis, single mode fibers can be used to convey light along portions of the object and reference arms as well as light traveling to and from the light source and the detector. However, bending motions as well as temperature fluctuations can produce optical path length variations in the fibers and dispersion that reduce measurement accuracy. Fiber optic cables to articulated optical probes are particularly susceptible to such disturbances when they use separate transmit and receive fibers.
In addition, object beam intensities are subject to change depending on the reflectivity characteristics of the measured test objects including different portions of the same test object. Imbalanced intensity variations between object and reference beams reaching the detector tend to reduce interference phase contrast, which can also reduce the accuracy with which the interferometric based measurements can be made.